ABSTRACT
The ancestors of all living chordates are to be sought among a group of strange fossils found in marine rocks of Cambrian to Middle Devonian age (Jefferies, 1979, in press a, in press b; Jefferies & Lewis, 1978). These show definite echinoderm affinities but are best regarded as primitive chordates, called calcichordates because they retain a calcite skeleton of echinoderm type. The calcichordates, contrary to my earlier interpretation, are not a subphylum but rather an “improper stem group” in Hennig’s sense (1969). Within them stem chordates, and also stem acraniates, stem tunicates and stem vertebrates can be recognised. The calcichordates are divided into two traditional groups - the cornutes, and the mitrates which were descended from cornutes.
